It is known that both inorganic and organic gaseous components in exhaled breath may provide useful insights into metabolic processes in certain diseases. The concentrations of such components have been studied in great detail in both research and clinical settings. Based on these insights, the concentration values aid in the establishing of a diagnosis, and have proven useful to monitor the well being of a patient, etc. Examples of gaseous components present in exhaled air include nitrogen monoxide, here referred to as nitric oxide (NO), carbon dioxide (CO2), oxygen (O2), and volatile organic compounds. One illustrative example is NO, which since it was found to be a diagnostic marker of inflammation in early 1990, has become the focus of much research. Different techniques and sensors have been suggested for use in the determination of NO concentration. Examples include, but are not limited to chemiluminescence, semiconductor-based sensors, electrochemical sensors, and polymer-based sensors.
Regardless of the identity of the gaseous component to be determined, or the technology chosen, it is important to be able to test the function, accuracy and reliability of the device.
Traditionally, reference gases are used, the reference gas being a gaseous mixture having a known and for practical purposes stable concentration of the substance to be measured by the measuring device. For example, if the measuring device is used to measure the concentration of NO in exhaled breath, a special reference gas with a known concentration of NO in nitrogen may be used. That is, the bulk gas is nitrogen which contains a specified concentration of NO. This reference gas is stored in compressed form in a gas cylinder and is then fed to the measuring device, often via a pressure regulator and a gas fitting. The reading of the measuring device is then compared with the known concentration of the substance to be measured in the special reference gas.
This technique is frequently rather expensive because of the cost of manufacturing, storing and handling the special reference gas. It is, in some cases, also technically difficult to manufacture the special reference gas within the required specifications. This is for example the case when a special reference gas with a low concentration of the substance to be measured is needed. It is also time consuming to order, transport and handle the special reference gas, which in addition may have limited shelf life. Further, the handling of the gas cylinder itself is inconvenient because of its size and weight.
Some measuring devices use pre-calibrated sensors or are otherwise pre-calibrated during manufacturing and need no calibration during their specified life-time. It would still be advantageous to have the possibility to test if the measurement value from a specific device is correct and reliable.